Generally, a conventional automatic transmission for a vehicle includes a torque converter, a multi-stage gear shift mechanism connected to the torque converter, and a plurality of friction elements actuated by hydraulic pressure for selecting a gear stage of the gear shift mechanism.
In such hydraulic control system, hydraulic pressure generated by a hydraulic pump is selectively supplied to each friction element by a plurality of control valves such that automatic shifting is realized in accordance with a driving state of the vehicle and engine throttle opening.
The above hydraulic control system generally comprises a line pressure controller for regulating hydraulic pressure generated by the hydraulic pump to line pressure, a damper clutch controller for actuating a damper clutch of the torque converter, a reducing pressure controller for reducing line pressure, a range controller for selectively supplying line pressure to lines corresponding to respective shift ranges, a shift controller for supplying hydraulic pressure from the range controller to lines corresponding to respective shift ranges, a hydraulic pressure controller for converting hydraulic pressure from the range controller to operational pressure operating the friction element, and a hydraulic pressure distributor for determining a hydraulic flow path corresponding to each shift range by operating valves according to the hydraulic pressure from the shift controller, and suitably distributing the operational pressure to each friction element.
The shift controller operates spool valves of the hydraulic pressure distributor according to a transmission control unit such that determines the hydraulic flow path corresponding to each shift range to realize shifting.
When changing shift stages, the timing of exhausting hydraulic pressure from one set of friction elements and supplying hydraulic pressure to another set of friction elements through the hydraulic flow path determined by the shift controller in accordance with each shift range and a change in hydraulic pressure levels in the hydraulic flow path greatly influences shift quality and durability of the automatic transmission. This mis-timing can also cause an abrupt increase in engine revolutions or locking of the shift mechanism and a change in hydraulic pressure levels decreases durability of the automatic transmission.
In order to improve shift quality and durability of the transmission by accurately controlling the timing of pressure supply and reducing a change in hydraulic pressure levels, a method of modifying shift valve structure has been developed. However, such a method complicates both the structure of the shift valves and the control process.